Remove uglify-es/uglify-js, use terser

[MattIPv4]

It looks like uglify-es has been deprecated on NPM. Further, terser seems to a more common tool used for compressing JS assets, so we should replace uglify-js with it.

Hopefully this may also resolve issues seen with newer syntax such as top-level await (https://github.com/cdnjs/packages/runs/5103276658?check_suite_focus=true), and issues with bad compression of libraries (cdnjs/packages#1363).

[rbubley]

There are other examples of bad compression with uglify-js resulting in broken libraries: see, e.g. firebase/firebase-js-sdk#5962 (comment)